1. Field of the Invention
The present invention relates to a vibration wave motor for driving a movable member by a travelling vibration wave generated on a vibration member, and more particularly to the structure of an anti-rotation lever of the vibration member.
2. Related Background Art
A vibration wave motor translates a vibration motion, created when a periodic voltage is applied to an electrostrictive (or piezoelectric) element functioning as an electromechanical energy transducer, to a rotational motion or linear motion. Since it does not require winding, it is simpler in structure and more compact than a conventional electromagnetic motor, and a high torque is obtainable at a low rotation speed.
FIGS. 3 and 4 illustrate a principle of the drive of a prior art vibration wave motor. FIG. 3 shows generation of the vibration wave of the motor. Numeral 15 denotes a vibration member (which is usually mode of metal material), and two groups of electrostrictive elements 14a and 14b are arranged with an appropriate spacing, that is, to satisfy a phase deviation of .lambda./4 to a travelling vibration wave .lambda..
In FIG. 3, the vibration member 15 is one electrode of the electrostrictive elements 14a and 14b. and an A.C. voltage. EQU v=V.sub.0 sin w.sub.0 t (1)
is applied to one group of electrostrictive elements 14a from an A.C. power supply l6a, and an A.C. voltage EQU v=V.sub.0 sin (wt.+-..pi./2) (2)
having a phase shift of .lambda./4 is applied to the other group of electrostrictive elements 14b through a 90.degree. phase shifter 16b so that the travelling vibration wave is generated.
Signs+and-in the formula (1) represent the directions of movement of the movable member 17 and it is selected by the 90.degree. phase shifter 16b as required. In FIG. 3, the 90.degree. phase shifter 16b has been switched to the - position and the voltage v=V.sub.0 sin (wt-.pi./2) is applied to the electrostrictive elements 14b.
A relationship between the applied A.C. voltage and the generated vibration wave when the A.C. voltage is applied to the electrostrictive elements either singly or in parallel, is explained. Let us assume that the predetermined A.C. voltage (i.e. v=V.sub.0 sin wt) is applied only to the electrostrictive element 14a. A vibration in the form of a standing wave as shown in FIG. 3(a) is generated in the vibration member 15. When the voltage v=V.sub.0 sin (wt-.pi./2) is applied only to the electrostrictive element 14b, a vibration in the form of a standing wave shown in FIG. 3(b) is generated in the vibration member 15.
On the other hand, when the two A.C. voltages having a phase shift are simultaneously applied to the electrostrictive elements 14a and 14b, the vibration wave travels. The change in time of the travelling vibration wave is shown by FIGS. 3(i).about.3(iv). FIG. 3(i) shows the vibration wave when t=2n.pi./w, FIG. 3(ii) shows the vibration wave when t=.pi./2w+2n.pi./w, FIG. 3(iii) shows the vibration wave when t=.pi.w+2n.pi./w, and FIG. 3(iv) shows the vibration wave when t=3.pi./2w+2.pi./w. A wave front of the vibration wave travels in an x-direction.
The travelling vibration wave thus generated accompanies a longitudinal wave and a lateral wave. More specifically, at any point A on the vibration member 15, it makes a rotating elliptic motion counterclockwise with a longitudinal amplitude u and a lateral amplitude w. Since the movable member 17 is press-contacted to the surface of the vibration member 15 which vibrates as described above, drive forces by the longitudinal amplitude u component of the elliptic motion are applied to the movable member 17 from apexes A, A', . . . of the vibration member by the contact (actually surface contact with a certain width) of the vibration plane of the vibration member and the apexes so that the movable member 17 is moved in a direction N (or rotated if the movable member is a rotor).
If the phase is shifted to 90.degree. by the 90.degree. phase shifter 16b, the vibration wave travels in a direction of -x and the movable member 17 is moved oppositely to the direction N.
Since the vibration wave motor frictionally drives the movable member by the vibration of the vibration member, the vibration member is usually resonated in a desired vibration mode so that a large vibration is generated in the vibration member.
A support for the vibration member in the vibration wave motor is usually constructed by merely press-contacting it to a vibration absorbing member such as felt in order to reduce the vibration load of the vibration member as much as possible. With such a construction, however, if the movable member such as a rotor has a heavy load, the vibration member counteracted by the drive of the movable member may be moved oppositely to the drive direction of the movable member. For example, if the above happens in a circumstance where the vibration wave motor is incorporated as drive means for a lens body tube of a one-lens reflex camera, the vibration member deviates from a reference position and control of focus by the vibration wave motor cannot be precisely done.
Several proposals to resolve such a problem have been made (for example, JP-A-201685/1984). In one proposal, the vibration member (a ring-shaped stator) has a comb-shaped contact, an anti-rotation lever (pawl) extending from an appropriate stator is fitted into a comb-shaped slit at a node (or loop) position of a standing wave which would be generated when the A.C. voltage is applied to only one of the electrostrictive elements 14a and 14b.
With the vibration wave motor of such a structure, the anti-movement means of the movable member is attained but another problem explained in FIG. 5 is occurs.
In a vibration wave motor shown in FIG. 5(a), a vibration member 18 comprising a resilient member 11 having a plurality of slits 14 and a piezoelectric element 12 attached to a bottom surface thereof, and a plurality of anti-rotation pawls 13 extending from a stator are fitted to the vibration member. Contact points of the plurality of pawls 13 and the vibration member are designated by P.sub.1 '.about.P.sub.4 '. In the illustrated example, the contact points P.sub.1 '.about.P.sub.4 'are arranged at each mode Q' defined when it is assumed that the vibration wave generated in the vibration member is a standing wave. Accordingly, the spacing l between the contact points P.sub.1 '.about.P.sub.4 ' is equal to 1/2 of a wavelength .lambda. of the vibration wave. This is shown in FIG. 5(b). In this vibration wave motor, the vibration wave motor actually generated is the travelling wave as explained in FIG. 3(i).about.3(iv). Accordingly, as the vibration wave travels, the contact points P.sub.1 '.about.P.sub.4 come to the position corresponding to a loop R' of the standing wave as shown in FIG. 5(c). This means that even if the contact points P.sub.1 '.about.P.sub.4 ' are arranged at each loop position of the standing wave, the result is substantially same as the case of FIG. 5.
However, if the vibration wave motor has the structure described above, the pawls 13 function as anti-rotation means and also serve as members to prevent vibration of the vibration member 18. Thus, the vibration generated in the vibration member is adversely affected by the pawls which are vibration prevention members. In other words, the pawls cause to be generated a discontinuity point of the vibration wave, and the larger the resistance (load) to the vibration, the higher is the rigidity of the resilient member 11 of the vibration member and the higher is a resonance point of the resilient member. This is a hazard to efficient drive of the vibration wave motor.
The following has been clarified by the inventors of the present invention.
The influence of the vibration resistance by the contact of the anti-rotation means to the vibration member changes with the travel of the travelling vibration wave, and the resonance point of the vibration member changes between FIGS. 5(b) and 5(c). On the other hand, when the vibration wave motor is driven at a high speed, the drive frequency of the motor is constant. As a result, a temporary out-of-resonance point phenomenon occurs and the output is reduced accordingly. The reason for the change of the vibration resistance by the contact of the anti-rotation means is as follows. Assuming that the travelling vibration wave is in the state shown in FIG. 5(b), the minimum amplitude position of the vibration wave corresponds to the contact point with the anti-rotation pawl, and the resistance by the pawl is very small. On the other hand, when the travelling vibration wave is in the state shown in FIG. 5(c), the maximum amplitude position of the vibration wave corresponds to the contact point with the anti-rotation pawl and the resistance of the pawl is very large. As a result, the change of the resistance (load) at the discontinuous point of the vibration wave which is the contact point with the pawl leads to a change of the resonance point which causes the reduction of output.